Pressure sensor chips can be used in a variety of applications to sense and measure pressure. In some applications, a pressure sensor chip must be relatively small in order to fit within a space in which pressure needs to be measured. For example, pressure sensor chips used in catheters can measure pressure of a fluid surrounding a catheter. The catheters can be used for pressure measurements in medical applications. For example, blood pressure inside a heart can be monitored using a catheter, such as during complex surgeries or heart tests. The catheters can be used in industrial applications to measure fluid pressure, e.g., gas pressure or liquid pressure. Pressure sensor dies for catheter applications, also referred to herein as a “catheter pressure sensor die” and a “catheter die,” must have a size smaller than an inner diameter of the catheter in order to be able to fit within the catheter's passageway. Long wires are located inside the catheter for signal transfer from the pressure sensor die to an external device.
One problem with traditional pressure sensor chips is that the smaller a pressure sensor chip, the more difficult it is for the pressure sensor chip to accurately sense pressure since the chip has a smaller pressure sensing element (e.g., diaphragm) and, therefore, has lower sensitivity to applied pressure. By having a smaller size, the pressure sensor chip has less room for wire connectivity thereto, which can mean fewer wires (e.g., three or less) connected thereto, a lower sensitivity to applied pressure, a higher signal to noise ratio, and/or yield loss at assembly due to difficulties with wire connection. The chip can therefore provide less accurate pressure measurements and can have a higher cost.
Another problem with traditional pressure sensor chips is that in applications using long wires, such as in catheter applications, the wires have to be connected securely to the chip such that the wires do not become loose and/or detached during assembly and/or during use. In order to simplify manufacturing and to provide enough wire connection strength, some traditional pressure sensor chips are placed on a substrate/die holder. A chip and substrate/die holder is also referred to herein as a “sub-assembly.” Short wires can be connected to the pressure sensor chip and the substrate/die holder to help ensure that the wires do not become loose or detached from the pressure sensor chip. Long wires attached to the substrate/die holder can connect the sub-assembly to an external device.
FIG. 1 shows one example of a traditional pressure sensor chip 176 on a substrate/die holder 178 with wires 180, 182 connected thereto via wire bonds. Ends of the wires 180, 182 are wire bonded to the chip 176, and portions of the wires 180, 182 near the ends are wire bonded to the substrate/die holder 178. The connection to the substrate/die holder 178 helps absorb any force applied to the wires 180, 182 so that the wire ends attached to the chip 176 can remain securely attached to the chip 176. However, including the substrate/die holder 176 increases an overall size of the sub-assembly, which can make it more difficult to use the sub-assembly in applications where small size is important. For example, including a substrate/die holder in catheter applications means that the catheter must have a diameter large enough to accommodate the catheter die and the substrate/die holder. FIG. 2 demonstrates this adverse affect by showing the chip 176 and the substrate/die holder 178 within a passageway 184 of a catheter 186.
Another problem with traditional pressure sensor chips is that wires connected to the chip via wire bonding each require the use of a bond pad having at least a minimum size and a minimum spacing between the bond pads. The minimum size is related to a diameter of the wire used for wire bonding, and the minimum spacing between bond pads is related to the size of tooling used at wire bonding. The bond pads can therefore require that the pressure sensor chip have a certain minimum size, namely a certain minimum width, which may still be too large for certain applications. If bond pads are made too small, the reliability of the wire bonds decreases. Wire bonding also causes a loop of wire to extend above the bond pads on the pressure sensor chip's surface, which can require additional space inside the catheter passageway and, therefore, increase a minimum inner diameter of the catheter and, hence, an overall size of the system. For example, FIGS. 1 and 2 show loops caused by the wires 180, 182 that are wire bonded to the chip 176. FIG. 2 also demonstrates how the catheter 186 must be sized to accommodate the loops.
Additionally, if a pressure sensor chip includes components for sensing other parameters such as temperature and pH, bond pads and wires for these other components can occupy valuable chip real estate. Real estate limitations on the chip make it difficult to accommodate a required number of bond pads on the chip.
Accordingly, there remains a need for improved semiconductor sensor chips.